1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an image display device which uses this liquid crystal display device.
2. Description of the Related Art
Liquid crystal display devices have been popularly used because of their characteristics that they are thin in configuration and exhibit the low power consumption. Particularly, a liquid crystal display device having active elements has a function of selectively giving potentials to respective pixel electrodes and holding such potentials and hence, the liquid crystal display device exhibits superior images compared with a liquid crystal display device of a type which has no active elements. Accordingly, the liquid crystal display devices of an active element type have been popularly used.
Further, as an image display device, an image display device which uses a so-called cathode ray tube has been known. Similarly, an image display device which uses a liquid crystal display device has been also known. The latter image display device exhibits less flickering compared with the image display device which uses the cathode ray tube and hence, images provided by the liquid crystal display device are gentle to human eyes. As the image display device using such a liquid crystal display device, versatile image display devices including liquid crystal monitors, notebook type personal computers, liquid crystal television sets, liquid crystal integral type personal computers PDAs and the like have been commercialized.
However, as a result of studies that inventors of the present application have extensively carried out, the inventors have found a new task that, with respect to the liquid crystal display device having active elements, when the operation is stopped, that is, when the supplying of power from the outside is stopped and thereafter the liquid crystal display device is again shifted to the operational state, there exists a case that a so-called flickering, that is, the strong unsteady shining of the screen appears.
The inventors also have found that this phenomenon is noticeable when the time counted from the stop of supplying of power to the supplying of power again is relatively short.
The inventors also have found that the above-mentioned phenomenon is further noticeable when the liquid crystal display device adopts a constitution in which an insulation layer is interposed between pixel electrodes and an orientation film or a constitution in which pixel electrodes and reference electrodes are provided on the same substrate and an insulation layer is interposed between layers forming these pixel electrodes and the reference electrodes.
A typical example of advantages brought about by the use of the liquid crystal display device in place of the cathode ray tube in the image display device is that the image display device exhibits the least flickering in addition to the previously-mentioned thin configuration and the low power consumption. However, the inventors have found that when the time for interruption of the supplying of power to the liquid crystal display device and the time for supplying power to the liquid crystal display device again in the image display device are short, even in the image display device using the liquid crystal display device, there exists a case in which the flickering is generated for several seconds to several 10 s seconds immediately after power is supplied again. This gives rise to a crucial task that the liquid crystal display device may loose one of advantages thereof and hence, the inventors have made efforts to solve the phenomenon of this task and to cope with the task.
As a result of our efforts, we have found that following phenomena which will be explained in detail are main causes of the task.
In the liquid crystal display device having active elements, when selection potentials for making the active elements have the ON state are applied to scanning signal lines, the potentials are selectively written in the pixel electrodes and, for the most of the time, non-selection potentials for making active elements have the OFF state are applied to the scanning signal lines so that the voltage applied in the ON state is held. The reason that the active elements are in the OFF state in most of the time is that since the liquid crystal display device usually sequentially and selectively drive a plurality of scanning signal lines, in the liquid crystal display device which corresponds to XGA having at least 768 scanning signal lines, for example, it is a general driving method that the time in which the OFF state is selected is (768-1) times longer than the time in which the ON state is selected.
Further, to prevent the deterioration of the liquid crystal material, the liquid crystal display device usually converts the potential applied between the pixel electrodes and the reference electrodes into an alternating current so as to prevent the direct current voltage from being continuously applied for a long time. However, this advantageous effect is merely obtained by inverting the polarity of the potential applied between the pixel electrodes and the reference electrode per one or a plurality of unit frames and hence, the effect only aims at the prevention of the applying of direct current voltage as the average for a long time. Accordingly, the fact that the substantially fixed voltage is applied to the pixel electrodes is not changed when viewed per each unit frame.
Further, the drive to invert the polarity of the potential applied between the pixel electrodes and the reference electrode per one or a plurality of unit frames can be performed only when power is supplied to the liquid crystal display device. That is, after such supplying of power is stopped, the applying of the approximately fixed potential to the pixel electrodes is continued. Then, at a point of time that the pixel electrodes are held at the OFF state due to the active elements, the pixel electrodes of the liquid crystal display device to which the supplying of power is interrupted are held at the OFF state for a relatively long time so that the applying of the fixed potential to the pixel electrodes is continued for a long time.
On the other hand, the potential is usually directly supplied to the reference electrode without through the active elements which are provided to respective pixels and hence, contrary to the pixel electrodes, after the supplying of power to the liquid crystal display device is stopped, the reference electrode immediately reaches the GND potential.
As a result, in the liquid crystal display device having active elements, when the supplying of power to the liquid crystal display device is stopped, the direct current potential difference is applied between the pixel electrodes and the reference electrode for a long time and the pixels are charged to the direct current. Accordingly, it has been found that even when power is supplied to the liquid crystal display device again, the potential between the pixel electrodes and the reference electrode at this point of time is driven in a mode that alternating current signals are superposed on the remaining direct current potential so that the imbalance is generated with respect to the liquid crystal drive voltage between polarities thus generating the flickering.
Further, it is found that the following is the reason that the generation of flickering is noticeable when the time counted from the stop of supplying of power to the restarting of supplying of power is relatively short. That is, when the supplying of power to the liquid crystal display device is stopped and a long time elapses thereafter, the potential of the scanning signal lines is converged to the GND state so that the leaking of charge stored in the pixel electrodes is generated through the active elements although a leaking amount is minute. Accordingly, when power is supplied to the liquid crystal display device again after the charge stored in the pixel electrodes is completely leaked, since the holding of the above-mentioned direct current potential between the pixel electrodes and the reference electrode is dissolved, no flickering is generated. Accordingly, when the time counted from the stop of supplying of power to the restarting of supplying of power is relatively short, the flickering is recognized noticeable in appearance.
It is also found that when the orientation film is arranged over the pixel electrodes, the orientation film performs the function of trapping the charge so that the above-mentioned flickering phenomenon is worsened.
It is further found that when an insulation layer is interposed between the pixel electrodes and the orientation film or when the pixel electrodes and the reference electrode are formed on the same substrate and an insulation layer is interposed between the pixel-electrode forming layers and the reference electrode-forming layers, these layers perform the function of trapping the charge and hence, the flickering phenomenon is further worsened.
Particularly, with respect to the liquid crystal display device in which the insulation layer is interposed between the pixel electrodes and the orientation film or the pixel electrodes and the reference electrode are formed on the same substrate and the insulation layer is interposed between the pixel-electrode forming layers and the reference electrode-forming layers, such a liquid crystal display device has been known as a device which can realize the wide viewing angle and hence, the further development of the device is expected as a device used for a liquid crystal monitor or a liquid crystal television set while substituting for a cathode ray tube. The fact that the flickering characteristics is further worsened in the liquid crystal display device having such a constitution constitutes an extremely crucial problem.